Programming Serial(UART) in C or C++

Arun Kapur

I am new to the Omega but not new to programming. I have looked through the Omega docs and through this forum but I am not able to find any documentation on what I need to program and setup the serial port in C or C++ - an example would really help. I m sure this must be somewhere, I just can seem to be able to find it.

Thanks in advance for your help.

Maximilian Gerhardt

@Arun-Kapur Using the UART on the Omega2 should be like using any other Linux UART device. For starters, use the program screen or miniterm.py (opkg install python-pyserial) to use the device /dev/ttyS1 for UART1. For C code, search e.g. "linux C uart" or "linux C serial port" or see https://stackoverflow.com/questions/6947413/how-to-open-read-and-write-from-serial-port-in-c .

Documentation is present from the Onion team. That's the first Google result for "Omega2 UART". https://docs.onion.io/omega2-docs/uart1.html

Arun Kapur

IS there a UART library for programming in C or C++ and is there example code I can see to get an idea of how to program the UART. I see examples and libraries programming the Omega in C for I2C, PWM and etc. I am not interested in python - just in C or C++

Any help would be appreciated.

Thank you!

William Scott

@Arun-Kapur Suggest you take the advice of @Maximilian-Gerhardt and closely read the docs. Not explicitly stated is the UART is available as a character device in the file system. As such you can write to it and read from it in C(++) as with any other character device. IMHO, it's level of complexity, or lack thereof, does not warrant a separate library.

Chris Stratton

@William-Scott said in Programming Serial(UART) in C or C++:

@Arun-Kapur Suggest you take the advice of @Maximilian-Gerhardt and closely read the docs. Not explicitly stated is the UART is available as a character device in the file system. As such you can write to it and read from it in C(++) as with any other character device. IMHO, it's level of complexity, or lack thereof, does not warrant a separate library.

In actuality, correctly using the Omega's UART is a bit complex, requiring quite a bit of configuration, especially as the defaults are a bit unusually less appropriate than on many other systems.

Arun Kapur

@Chris-Stratton Would you be able to share any example code ?

Arun Kapur

@Maximilian-Gerhardt I read the Onion documentation you pointed me to and that is why asked the question. I don’t want to write scripts or to program in python as the examples show but do this in C or C++. Perhaps you can point me to the C or C++ libraries for the Omega for doing this.

Maximilian Gerhardt

@Arun-Kapur I don't see any problem with the C code I have pointed you to.

I have personally tested the C code to work as expected on my Omega2+ for UART1 on 115200 Baud (8N1 standard config, non-blocking).The receiving side is a USB-Serial converter on my Windows 10 system. Connections are already listed in the documentation (GND, RX1, TX1).

Code below.

#include <errno.h>
#include <fcntl.h> 
#include <string.h>
#include <termios.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

int
set_interface_attribs (int fd, int speed, int parity)
{
        struct termios tty;
        memset (&tty, 0, sizeof tty);
        if (tcgetattr (fd, &tty) != 0)
        {
                fprintf (stderr, "error %d from tcgetattr", errno);
                return -1;
        }

        cfsetospeed (&tty, speed);
        cfsetispeed (&tty, speed);

        tty.c_cflag = (tty.c_cflag & ~CSIZE) | CS8;     // 8-bit chars
        // disable IGNBRK for mismatched speed tests; otherwise receive break
        // as \000 chars
        tty.c_iflag &= ~IGNBRK;         // disable break processing
        tty.c_lflag = 0;                // no signaling chars, no echo,
                                        // no canonical processing
        tty.c_oflag = 0;                // no remapping, no delays
        tty.c_cc[VMIN]  = 0;            // read doesn't block
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        tty.c_iflag &= ~(IXON | IXOFF | IXANY); // shut off xon/xoff ctrl

        tty.c_cflag |= (CLOCAL | CREAD);// ignore modem controls,
                                        // enable reading
        tty.c_cflag &= ~(PARENB | PARODD);      // shut off parity
        tty.c_cflag |= parity;
        tty.c_cflag &= ~CSTOPB;
        tty.c_cflag &= ~CRTSCTS;

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
        {
                fprintf (stderr, "error %d from tcsetattr", errno);
                return -1;
        }
        return 0;
}

void
set_blocking (int fd, int should_block)
{
        struct termios tty;
        memset (&tty, 0, sizeof tty);
        if (tcgetattr (fd, &tty) != 0)
        {
                fprintf (stderr, "error %d from tggetattr", errno);
                return;
        }

        tty.c_cc[VMIN]  = should_block ? 1 : 0;
        tty.c_cc[VTIME] = 5;            // 0.5 seconds read timeout

        if (tcsetattr (fd, TCSANOW, &tty) != 0)
                fprintf (stderr, "error %d setting term attributes", errno);
}

/* use omega UART1 */
const char *portname = "/dev/ttyS1";
int uartFd = -1; 

void uart_writestr(const char* string) {
	write(uartFd, string, strlen(string));
}	

void uart_write(void* data, size_t len) {
	write(uartFd, data, len); 
}

ssize_t uart_read(void* buffer, size_t charsToRead) {
	return read(uartFd, buffer, charsToRead); 
}

int uart_open(const char* port, int baud, int blocking) {
	uartFd = open (port, O_RDWR | O_NOCTTY | O_SYNC);
	if (uartFd < 0)
	{
			fprintf (stderr, "error %d opening %s: %s", errno, port, strerror (errno));
			return -1;
	}
	set_interface_attribs (uartFd, baud, 0);  // set speed, 8n1 (no parity)
	set_blocking (uartFd, blocking); //set blocking mode
	printf("Port %s opened.\n", port); 
	return 1;
}

int main(int argc, char* argv[] ) {

	if(!uart_open(portname, B115200, 0)) 
		return -1;

	for(int i = 0; i < 10; i++) {
		printf("[+] Writing string to UART.\n");
		uart_writestr("Hello world!\n");
		usleep(1000 * 1000);
	}
	
	return 0;
}